Add http checking

[hobofan]

Currently we are only checking file:// links but checking http links is obviously a high priority.

Some things to consider:

• The results of http checks should probably be cached since it is very likely that the same link is used multiple times
• I don't think that it should be the default since even with a handful of links it is bound to be slow and the result flaky depending on the network connection.

[hobofan]

It might also might sense to have a "disallow-http" flag that disallows all links that are not on a whitelist, in order to ensure that the documentation completely works offline.

With #10 merged, this should be fairly straightforward, if we also pass a configuration struct through to all tests.

[hobofan]

Implemented in #22, but one thing is still missing for it to be ready for a release:

I don't think that it should be the default since even with a handful of links it is bound to be slow and the result flaky depending on the network connection.